Detection systems which are presently available are classified into a surge-receiving-type detection system and a pulse-radar-type detection system.
The surge-receiving-type system can be sorted into a power-line transmission system and a microwave radio transmission system.
A description will be given of the construction and operation of conventional surge-receiving-type detection system.
FIG. 1 illustrates the construction of a detection system of a power-line transmission line (generally referred to as a "B-type system").
A power line is cut due to a thunder-bolt at a point marked by 1 in this Figure. As a result of the cutting, a surge pulse is transmitted as indicated by 1.fwdarw.2 and 1.fwdarw.3. The surge pulse received at a subordinate station is sent to a master station through a route 4.fwdarw.5. It is therefore necessary that the transmission delay time t.sub.3 incurred during the surge pulse transmission 4.fwdarw.5 be exactly detected. In the master station, the time t.sub.1 at which the surge pulse is received at the electric station A and the time which is obtained by subtracting the time t.sub.3 from the time t.sub.2 +t.sub.2 of the surge pulse received at the electric station B, i.e., the time t.sub.2. and computes the distance between the station A (or the station B) and the point of cutting of the power line, in accordance with the result of the comparison. The computed distance is then delivered to, for example, an automatic printing device.
A description will be given of a detection system of the microwave radio transmission type.
FIG. 2 shows the construction of the detection system of the microwave radio transmission type (generally referred to as "micro-B-type").
This system is based on the same detection method as the power line transmission type system, but is different from the latter in that a microwave circuit is used as the means for transmitting the surge signal from a subordinate station to the master station.
In this system, 12 microwave circuits or so are exclusively used for this purpose.
On the other hand, the detection system of pulse radar type is as follows.
Namely, a detection pulse is transmitted through the power transmission line and the pulse reflected by the defective point is received so that the distance to the defective point is computed by measuring the time until the reflected pulse is received.
A description will be given of the drawback of the surge-receiving-type detection system.
The power-line transmission-type system, which is one of the surge-receiving-type system, has a drawback that it essentially requires a coupling device because this type of detection system utilizes the power transmission line a the detection circuit.
Another problem is that the exchange of the signal is impossible when the defect exists in the phase (one of three phases) used for the signal transmission. Thus, the probability of receipt of the signal is 2/3.
In addition, error tends to be caused in the detection because of noises and surges which affect the detection signal.
Furthermore, it is necessary that the transmission delay time be exactly grasped. (Slight difference in the transmission delay time occurs to cause an error)
In order that the detection is conducted within a tolerance of 1 km, the change in the transmission delay time has to be within about 3 .mu.sec as will be understood from the following equation. EQU Change in delay time= 1 km/300,000 km (velocity of light)=3 .mu.sec.
In addition, error tends to be caused in the detection time due to collapse of the waveform, because of the use of the analog signals. Namely, the waveform of the signal tends to be collapsed or deformed to make the rise of the signal dull particularly when the length of the transmission line is large, so as to make the detection difficult or to cause error in the detection.
Drawbacks of the detection system of the microwave transmission type are as follows.
It is necessary that a multiplexed microwave circuit for exclusive use has to be employed.
In addition, the rate of occupation of the circuits is large: namely, 12 circuits are exclusively used in general.
Furthermore, this type of detection system is difficult to maintain and requires a periodical check of the signal delay time which requires a special technique (certificate is necessary).
This type of system also suffers from the same problems caused by a change in the transmission delay time and by the collapse of the waveform, as in the case of the surge-receiving-type detection system.
Finally, the pulse-radar-type detection system suffers from the following problems.
Namely, waves reflected by stationary points such as the branches of the power transmission line are included. In addition, the detection pulse is attenuated particularly when the distance to the defective point is large. The detection, therefore, is rendered difficult and errors are often caused in the detection.
In addition, this system requires a coupling system for coupling a detection pulse to the power transmission line.
The pulse-radar-type detection system relies upon a principle which is fundamentally different from that of the detection system of the present invention, so that detailed description is omitted in regard to the construction of this type of system.